NO, 5 BEHAVIOR OF THE NIGHT MONKEY — MOYNIHAN 25 



with Gulps and/or Sneeze-grunts. They may have been produced by 

 weaker hostile motivation than typical Gruff Grunt performances and 

 have contained a relatively stronger escape component. (The differ- 

 ence in causation between these soft Grunts without Swaying and 

 silent Swaying with typical Gruff Grunts was not clear.) 



RESONANT GRUNTS 



Resonant Grunt performances are much more complex in form 

 than typical Gruff Grunt performances, and seem to be much rarer. 



All the Resonant Grunts heard were uttered in long series of at 

 least 10 to 15 notes. The speed, loudness, and pitch of the notes 

 varied considerably. 



The most frequently heard, and simplest, type of series was as fol- 

 lows. The first notes were soft and low, much softer than typical 

 GrufT Grunts but otherwise similar. Succeeding notes gradually be- 

 came louder and louder throughout the first half of the series. By 

 the middle of the series, they were often very loud indeed, much more 

 so than any Gruff Grunts, and had developed a penetrating, resonating 

 quality. They were still gruntlike (insofar as they retained a gutteral 

 "rrrr" sound), but they had also acquired an "ooo" sound slightly 

 reminiscent of the Hoots (see below). All the notes during the first 

 half of a series were always uttered exceedingly rapidly one right after 

 the other, in an emphatic "pumping" rhythm. The intervals between 

 successive notes were much shorter than the intervals between notes in 

 typical series of Gruff Grunts. It is possible that the pitch of the 

 successive notes gradually became higher during the first half of some 

 of these Resonant Grunt series, but this change (if it actually oc- 

 curred) was always slight. Once the climax of a series was reached, 

 the next few notes remained essentially similar, loud, resonating, and 

 rapid. Then, during the final quarter (approximately) of the series, 

 the rhythm usually slowed down a little and, at least in some cases, the 

 notes became successively lower and/or softer. 



Series of this type were heard only among the captive individuals 

 on Barro Colorado Island. One male uttered 30 to 40 series during 

 a violent dispute with neighbors in adjoining cages immediately after 

 being reintroduced into a cage with which he was thoroughly familiar 

 after an absence of several weeks. One female uttered one series 

 toward the end of a long and violent dispute with a male to whom she 

 had just been introduced. Another female, alone in her cage, uttered 

 several series after being disturbed by a human being, just after the 

 human being had left and disappeared from her sight. These three 

 individuals uttered their series of Resonant Grunts at night ; and they 



